1. Field of the invention
The present invention relates generally to a low pressure mercury vapor discharge lamp, and more particularly, to a low pressure mercury vapor discharge lamp having double layers on its inner surface.
2. Description of the Related Art
Low pressure mercury vapor discharge lamps are well-known, such as, for example, fluorescent lamps. In a typical fluorescent lamp, a fluorescent layer is coated on an inner surface of a glass tube defining a discharge space. The discharge space is filled with a rare gas, such as argon, along with mercury, as a discharge gas. A pair of electrodes are provided in the tube for generating a discharge in the tube.
When a voltage is applied between the electrodes, atoms of the mercury in the tube are excited and a discharge is generated in the tube between the electrodes. The excited mercury atoms emit ultraviolet rays which excite fluorescent materials of the fluorescent layer, thereby causing fluorescent materials emit a visible light.
Recently fluorescent lamps having high wall loads have been developed. Since these lamps have small diameter glass tubes, such as 10-15 mm in inner diameter, these lamps have high wall loads of more than 550 W/m.sup.2. The fluorescent layers of lamps with high wall loads are generally likely to decrease in luminous efficiency or in luminous flux early in their lives. The reasons or mechanism of the early decrease have not yet been determined.
U.S. Pat. No. 4,335,330 teaches that fluorescent materials absorb excited mercury atoms and react with the excited mercury atoms and that, as a result, a darkening of the fluorescent layer occurs which considerably reduces the luminous flux of the lamp.
However, when lamps have double fluorescent layers, some still exhibit an early decrease in luminous flux, even if the fluorescent materials employed are among the materials taught as being good in U.S. Pat. No. 4,335,330.